This invention relates to a contact lens that restores the ability to focus on objects within a range of distances near to the user (referred to as xe2x80x9cnatural accommodationxe2x80x9d), while retaining the ability to see distant objects. More specifically, this invention relates to a contact lens with a conventional spherical concave surface conforming to the curvature of the eye (base curve) and having a non-conventional convex surface (optic curve) combining spherical and non constant aspherical curvature resulting in an optical system that provides true monocular presbyopic correction (correction of presbyopia in each eye independently, instead of partial or full distance correction in one eye and partial or full near correction in the other) and restores the phenomenon of xe2x80x9cnatural accommodation.xe2x80x9d Additionally, the invention affords a methodology of fitting that substantially reduces the skill and experience required by the contact lens fitter to a very basic level while affording a high degree of clinical success and patient satisfaction.
Normally between the ages of 40 and 45, presbyopia or old sightlessness is brought about by loss of elasticity of the crystalline lens of the eye, causing blurred vision at near points due to the reduction of the ability of the eye""s natural lens to accommodate the changes in curvature necessary to focus on both near and distant objects.
When a person is free of presbyopia, the eye retains its full range of natural accommodation. This type of person""s vision can be corrected by eyeglasses or contact lenses providing only the correction required for distance vision, and natural accommodation would automatically provide correction for near and intermediate distance vision.
For the contact lens wearer who requires presbyopic (or near vision) correction, in addition to distance correction, a variety of options have been available. These individuals may be fitted with single vision contact lenses corrected for distance, and wear reading glasses for near correction. Another alternative is to provide a contact lens for one eye that is corrected for distance vision and to provide a contact lens for the other eye that is corrected for near vision (this practice is referred to as monovision because only one eye is corrected for near vision), or the fitting of bifocal or multifocal contact lenses.
During the 1950""s, a variety of contact lenses were designed for the correction of presbyopia. Theme contact lenses, although very innovative in design, met with only limited success because the only readily available material was Poly Methyl Methacrylate (Plexiglass), also known as PMMA which does not transmit oxygen. As bifocal and multifocal designs of the period were quite thick and heavy compared to conventional distance correction contact lenses, these presbyopic contact lenses were uncomfortable to wear for substantial periods of time. Additionally, the fitting of these bifocal and multifocal contact lenses required considerable time and skill on the part of the contact lens fitter.
During the 1970""s, both soft contact lenses and rigid gas permeable (RGP) contact lenses were introduced with the availability of these new materials, renewed enthusiasm brought about several new designs for contact lenses for the correction of presbyopia.
RGP materials provide oxygen transmission through the lens material itself, and afforded new hope for the earlier designs developed in PMMA material. However, lens thickness and resultant patient discomfort continued to be a problem.
One of the early benefits recognized with soft contact lenses was the comfort and ease of fitting and, for this reason, by 1995 approximately 85% of new contact lens wearers are being fitted with soft contact lenses. As soft contact lenses command such a large share of the contact lens market, it is natural that considerable effort would be made to develop bifocal and multifocal contact lens designs in soft contact lens material.
There are two types of contact lens designs for the correction of presbyopiaxe2x80x94Alternating (or Translating) and Simultaneous.
(1) In the alternating (or translating) vision technique, the lenses are very similar in design to bifocal eyeglass lenses in that the wearer sees through the distance segment in the upper portion of the lens when looking straight ahead and sees through a lower near vision segment when the eye (moves) to look down. Alternating vision lenses have proven to be successful in RGP designs, but have met with little success when designed in soft contact lenses.
Perhaps the reason that alternating vision soft contact lens designs were not as successful as the same design concept in RGP materials was because lens translation is necessary for this design to be successful. The translation from distance to near is achieved through the mechanical action of the lens resting on the lower eyelid and, when the eye looks down, the lens remains stable on the lower eyelid causing the pupil of the eye to translate from the distant vision portion of the lens to the near vision portion of the lens. Soft lens material by its nature caused this modality to fail as there was insufficient rigidity in the soft lens to remain properly positioned on the lower eyelid and often the lens would slip underneath the lower eyelid during translation.
(2) Simultaneous vision bifocal or multifocal contact lenses are either concentric or aspheric in design with focal power changing through different areas of the lens. Lenses are fitted so that distance, intermediate and near zones focus images simultaneously on the retina of the eye and the brain then separates out the image desired.
Theoretically, with adaptation, the ability to change focus naturally from near to far with no blurring in between can be achieved with simultaneous vision lenses in both RGP and soft contact lenses.
As alternating presbyopic designs proved to be unsuccessful in soft contact lens designs, most of the development work with soft contact lenses was done in the area of simultaneous presbyopic correction with concentric designs or aspheric designs.
During the 1980""s, several designs of concentric and aspheric soft contact lenses were introduced. Soft aspheric multifocal contact lenses typically provided relatively weak reading addition power and therefore worked beat in early presbyopia.
Reading addition powers are referred to by eye care professionals as xe2x80x9caddxe2x80x9d power, and represent the difference between the distance correction and near correction prescribed by an eye care professional for eyeglasses or contact lenses. Accordingly, a prescription of xe2x80x9cxe2x88x923 with a +2 addxe2x80x9d (which would be typical for moderate presbyopia) would mean that distance vision requires xe2x88x923 diopters of correction, and near vision requires an additional 2 diopters of plus correction, resulting in xe2x88x921 diopters of near vision correction. In conventional monovision, the dominant eye would be fitted with a xe2x88x923 distance correction lens, and the other eye would be fitted with a xe2x88x921 near correction lens.
This type of solution is often satisfactory in early presbyopia because the user still has some remaining visual accommodation and the needed add power is usually between +0.75 and +1.25, which is usually low enough for the brain to comfortably select the desired image in most people. However, conventional monovision becomes less satisfactory as, presbyopia becomes more advanced because the needed add power increases and visual accommodation has deteriorated further, so that the visual imbalance exceeds the brain""s ability to select the desired image from the appropriate eye.
Typically, early presbyopes, would be between the age of 40 and 45, and would require add power of between +1.00 and +1.50 diopters. Moderate presbyopes would usually be between 45 and 55 years and would require add power of between +1.50 and +2.00 diopters. Mature presbyopes would usually be older than age 55 and require an add power of between +2.00 and +3.00 diopter.
The add corrective power of current aspheric multifocal contact lens designs is usually limited to only +0.75 to +1.25 diopters because the brain must be able to separate out the desired image (and also suppress the undesired images) from the multiple images (near, intermediate or distant) being simultaneously focused by the multifocal contact lens design. In order to achieve this suppression, the images cannot be too different from each other. However, if aspheric corrections are increased in attempts to achieve higher add powers, the images become too different for the brain to suppress the undesired images, resulting in blurred vision. Even at add powers of +0.75 to +1.25 diopters, many patients suffer some blurring or ghosting with multifocal contact lens designs because their brains are not able to completely separate the desired image while simultaneously completely suppressing the undesired images.
Some contact lens fitters may attempt to use aspheric designs to achieve near distance correction of up to +2.00 diopters (or more) by undercorrecting the distance vision of the non-dominant eye by between 0.25 and 1.00 diopters, thereby theoretically providing up to +2.00 diopters (or more) of near vision correction, instead of the +0.75 to +1.25 diopter correction that would be provided if that eye had been fully corrected for distance vision with an aspheric multifocal contact lens. The dominant eye would be corrected to maximum distance acuity in such a situation. However, this creates even more blurring and ghosting. This technique is called modified monovision.
Aspheric optics have been incorporated on both the front and back surfaces of soft contact lenses. However, it is believed that front surface aspherical multifocal soft contact lenses provide better presbyopic correction. Still, only limited success is achieved because providing add power of +0.75 to +1.25 (or more) usually results in reduced distance acuity. For this reason, many contact lens fitters find it necessary, when using aspheric soft multifocal contact lenses, to undercorrect the distance power in one eye to improve near vision, while correcting the other eye fully for distance vision, as discussed above. When attempting to fit moderate to mature presbyopes, this modified monovision almost always results in a visual compromise similar to that of conventional monovision.
Concentric multifocal lens designs have an advantage over aspheric designs in the fitting and correcting of more mature presbyopes requiring add power of more than +1.25 diopters, primarily due to the availability of higher add power correction and central power zones of different diameters. Concentric soft multifocal contact lenses have been made with the central distant correction zones and central near correction zones in the latter designs, the central power zones would be corrected by the amount prescribed to correct near vision. It is believed that central near add zones have been more successful at correcting presbyopia than central distance zones, when incorporated in concentric multifocal soft lens designs. Although concentric center add multifocal designs have the ability to correct higher add power requirements, most individuals fitted with this type of lens experience moderate to significant amounts of visual discomfort due to ghosting of images or a 3-D effect, at near distances. These effects diminish with adaptation, but still cause a high portion of wearers to discontinue the use of this type of presbyopic contact lens.
The reality of the existing art of presbyopic correction with simultaneous vision contact lenses is that no currently available lens system, be it aspheric or concentric, provides monocular multifocal correction for moderate to mature presbyopia. In most cases, some form of modified monovision is required in an attempt to satisfy the visual requirement for near and far vision. To this end almost all currently available presbyopic contact lens manufacturers indicate in their fitting manuals the requirement of compensating one eye more for near and the other eye more for distance correction. This is the norm rather than the exception. Additionally, no currently available multifocal contact lens has the ability to restore the phenomena of natural accommodation and successful results are difficult to achieve and require considerable time and experience on the part of the fitter.
It is therefore an object of this invention to provide true multifocal correction for moderate and mature presbyopes requiring up to +3.00 diopters of add power without the need to compensate one eye for near and the other eye for distance.
It is a further object of this invention to provide rapid patient adaptation with minimal initial visual discomfort.
It is a still further object of this invention to provide a presbyopic optical system that restores the phenomenon of natural accommodation.
It is a still further object of this invention to provide a system of fitting and methodology that allows a contact lens fitter with little or no multifocal contact lens fitting experience to achieve a very high degree of success and patient satisfaction.
These and other objects are achieved by a contact lens having a central circular region (an xe2x80x9caccommodation zonexe2x80x9d or xe2x80x9csweet spotxe2x80x9d named zone 1) that in overcorrected for near vision, and that is small enough that it does not impair distance vision. Preferably, a plurality of concentric transition region (or rings), optimally two (named zone 2 and zone 3, progressing radially outwardly), are provided between the sweet spot and the outer region (or ring) of the lens (named zone 4), which 8 corrected for distance vision. Preferably, the sweet spot has a diameter of between approximately 1.0 millimeters and approximately 2.5 millimeters, preferably between approximately 1.5 millimeters and approximately 1.9 millimeters, and optimally either approximately 1.5 millimeters or approximately 1.9 millimeters, Preferably, the transition rings (zones 2 and 3) are each approximately 0.5 millimeters wide. Preferably also, the remaining portion of the lens (zone 4) extends radially outward from the outermost transition ring to at least approximately 8 millimeters. Because the human pupil cannot expand beyond approximately 8 millimeters in diameter, the portion of the lens extending more than approximately 8 millimeters radially outward from the center is not an optical portion and functions only as a carrier.
Preferably, the sweet spot is spherical and is overcorrected by between 25% and 100% over the near vision correction prescribed for the user. Preferably, the remaining optical portions of the lens are aspheric, with different diopter shifts over different regions. Optimally, for high add power, zone 2 provides a diopter shift of approximately 1.6 diopters, zone 3 provides a diopter shift of approximately 1.2 diopters, and zone 4 provides a diopter shift of approximately 0.9 diopters. For low add power, optimally zone 2 provides a diopter shift of approximately 0.1.1 diopters, zone 3 provides a diopter shift of approximately 0.8 diopters, and zone 4 provides a diopter shift of approximately 0.6 diopters.
The contact lens manufacturing lathe disclosed in the example below provided contact lenses that achieved the desired results. However, some experimentation may be necessary to achieve the desired result with different equipment, but this experimentation should not be undue.
The invention incorporates both concentric and aspheric design principles and can be produced with a high add power correction or a low add power correction. In addition, the lens system offers two accommodation zone diameters for different sized pupils to achieve maximum near point acuity without reduction in distance visual acuity.
The higher add power lens has a power transition of 3.7 diopters across the usable optic zone, and the low add power lens has a power transition of 2.6 diopters across the usable optic zone.
The accommodation zone should cover approximately 50% of the pupil area for maximum success in distant, intermediate and near visual acuity. The accommodation zone functions to restore the phenomenon of natural accommodation by creating a very small area of over magnification in the center of the pupil of approximately 25% to 100% over the near vision correction required by the indicated reading add power. Surprisingly, distance vision will not be substantially impaired if the accommodation zone covers 50% or less of the pupil area. Further, the function of natural accommodation will be restored to an unexpectedly great extent.
Although the inventor is not sure (and the validity and enforceability of any patent issuing hereon shall not be affected by the accuracy or inaccuracy of this explanation), the inventor believes that, in near vision, a user""s pupils constrict, so that the accommodation zone occupies a largo enough portion of the pupil area for the accommodation zone to become effective. Normal reading correction is prescribed for approximately 15 inches (approximately 38 centimeters). Accordingly, the overcorrection of the accommodation zone (sweet spot) allows the user to ace from 8 inches to 15 inches, thus restoring the function of natural accommodation. In distance vision, however, the pupil will be normally dilated, so that the accommodation zone is small enough that the brain ignores the image generated by it. The constriction of the pupil for near vision is known as xe2x80x9caccommodative pupil response.xe2x80x9d
The accommodation zone is blended to the distance zone 4 via two zones of non constant aspherocity which allows true monocular correction of near, intermediate and distant vision. Near vision correction, when tested at the standard distance of approximately 15 inches (approximately 38 centimeters) offers normal best corrected acuity and when reading material is brought closer to the eyes, up to about eight inches (approximately 20 centimeters), near acuity remains stable and often improves due to the increased near power created by the sweet spot.
Due to the non constant aspheric transition from the sweet spot to zone 4, adaptation problems associated with prior designs of concentric or aspheric multifocal contact lenses are substantially reduced or eliminated completely.
Historically, the fitting of multifocal contact lenses has been more an art than a science as the variables associated with fitting presbyopic contact lenses are considerable. Often success has only been achieved through the process of trying many different lenses on the patient in the hope of finding a lens that generates a good presbyopic response. The contact lens fitter""s degree of experience in the fitting of multifocal lenses has also been a key to achieving a successful fitting with good visual results.
The fitting of lenses according to this invention requires accurate centering of the lens over the pupil of the eye in order to achieve the expected results. To determine the location of the sweet spot relative to the pupil is often difficult because the pupil may not be aligned with the center of the cornea or for other reasons. Thus, the invention also incorporates the use of a diagnostic trial lens with a white ring corresponding in diameter and location to the sweet spot. The exact position of the center of the contact lens can be determined and the relative position of the sweet spot to the pupil and the percentage of pupil covered by the sweet spot is easily observed. The use of the diagnostic lens allows the fitter to very quickly determine the proper sweet spot size, which increases the chances of successful fitting. For example, if the accommodation zone does not align within the pupil, the fitter knows that the standard lens design will not work and a custom lens design with an offset accommodation zone will be required.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the presently preferred embodiments for carrying out the invention and the accompanying drawings.